The present invention relates to a semiconductor device and a method for fabricating the same, and more particularly relates to a semiconductor device including a resistor and an MIS transistor and a method for fabricating the same.
An analog semiconductor device includes a resistor element formed of a resistive material, i.e., a polysilicon film, in addition to a CMIS transistor. In the analog semiconductor device, the CMIS transistor is required to be capable of operating at high speed and low power consumption. As for the resistor element, a resistor value thereof has to be controlled and stabilized (for example, see Japanese Laid-Open Publication No. 2003-152100).
In recent years, as the accuracy of resistors has been increased, fluctuation in the resistance value of a resistor due to auto-doping of an impurity during thermal treatment has become a serious problem. When a heavily impurity doped layer to be a source/drain regions of an MIS transistor is formed by ion implantation and then thermal treatment is performed to activate an impurity that has been ion-implanted, the impurity diffused outward from a surface of the doped region is diffused again in the resistor. This causes the fluctuation in the resistance value of a resistor. Now, a method which has been conventionally examined for the purpose of suppressing such a resistance value fluctuation will be described. FIGS. 8A through 8C are cross-sectional views illustrating respective steps for fabricating a semiconductor device including a NMIS transistor and a resistor in a conventional manner.
According to a known method for fabricating a semiconductor device, first, in the process step of FIG. 8A, a p-type well region 102 and a trench isolation 103 are formed in a semiconductor substrate 101. Thereafter, a gate insulation film 104 is formed on part of the semiconductor substrate 101 of which sides are surrounded by the isolation 103, i.e., an active region 100 and then a polysilicon film (not shown) is formed over the substrate. Thereafter, the polysilicon film is patterned so that a gate electrode 105a is formed on the gate insulation film 104 and also a resistor element 105b to serve as a resistor is formed on the isolation region 103. Then, using the gate electrode 105a as a mask, arsenic ions are implanted into the semiconductor substrate 101, thereby forming an n-type lightly doped layer 106. Subsequently, boron ions are implanted into the semiconductor substrate 101 using the gate electrode 105a as a mask, thereby forming a p-type pocket doped layer 107 in part of the semiconductor substrate 101 located under the n-type lightly doped layer 106.
Next, in the process step of FIG. 8B, a side wall 108 is formed on a side surface of the gate electrode 105a. At this time, the side wall 108 is also provided on a side surface of the resistor element 105b. Thereafter, arsenic ions are implanted into the semiconductor substrate 101 using the gate electrode 105a and the side wall 108 as a mask, thereby forming an n-type heavily doped layer 109.
Next, in the process step of FIG. 8C, an insulation film 110 is formed overt the substrate and then thermal treatment is performed to activate the impurity that has been ion-implanted.
According to the method described above, with the insulation film 110 having been formed, thermal treatment is performed to activate an impurity. Thus, auto-doping of the impurity from the n-type heavily doped layer 109 to the resistor element 105b can be prevented.
However, as in the known method, if with an upper surface of the substrate entirely covered with the insulation film 110, high temperature thermal treatment is performed to activate the impurity, breakdown of the gate insulation film 104 is caused. With reduction in the thickness of the gate insulation film 104, such breakdown of the gate insulation film 104 has become clearly noticeable.
On the other hand, when after the process step of FIG. 8B, thermal treatment for activating an impurity is performed without forming the insulation film 110, breakdown of the gate insulation film 104 is not caused. However, another problem arises, i.e., variation in the resistance value of the resistor due to auto-doping is caused.